Saturated vapor steam at 4 MPa is contained in a 2 m3 insulated vessel. A valve on the top of the tank is opened, allowing steam to escape. If any liquid is formed, it will drop to the bottom of the tank, so that saturated vapor only will escape from the top of the tank. Calculate the total mass that has escaped when the tank pressure reaches 1 MPa. Note that value of the saturated vapor enthalpy leaving the tank changes as the pressure drops. However, the values of hg at 4 MPa and 1 MPa are not grossly different, so the average of the two can be used for the escaping stream term in the energy balance, if needed.